1. Field of the Invention
This invention relates generally to bicycle gear shift systems having a cable actuated derailleur. More particularly, this invention relates to a cable pre-load and sealing system for a cable actuated derailleur.
2. Previous Art
Street and mountain bicycles ("mountain bikes") are typically equipped with gear shift systems having multiple gears for optimizing bicycle performance. Such gear shift systems are optimally adapted to operate with a high degree of precision and efficiency so that the time required to shift gears is minimized and bicycle performance is maximized. The gear shift systems typically include a series of freewheel sprockets, front and rear derailleurs, shift actuators and a control cable system.
The control cable system generally includes a central cable having a sheath which covers at least a portion of the control cable. Such control cables are commonly referred to as "Bowden type cables." The control cable is designed to slide axially and reciprocally with respect to the sheath.
In conventional derailleur-activated multiple-gear bikes, the rear derailleur is used to transfer the bicycle drive chain from one freewheel sprocket to another freewheel sprocket. The derailleur moves in response to displacement of the derailleur control cable. Pulling the control cable causes the rear derailleur to shift the drive chain to a larger and more inboard sprocket, producing a lower gear (downshifting). Releasing the control cable permits a cable-tensioning derailleur return spring to shift the drive chain to a smaller and more inboard sprocket, producing a higher gear (upshifting).
Bicyclists optimally desire smooth and rapid gear shifting. Minimizing the time required for shifting is a factor which affects shifting performance. During shifting, optimally, the chain is transferred from central alignment with one sprocket to central alignment with another sprocket. In practice, however, several derailleurs require overshift (i.e., briefly positioning the chain past central alignment of a sprocket during shifting) to move a chain from a smaller sprocket to a larger sprocket.
After overshift, the drive chain is returned to central alignment with the desired sprocket. Several systems have been employed to achieve "overshift return". For example, in some systems, overshift return is accomplished by manual readjustment of the shift actuator. In other systems the shift actuator includes an overshift return mechanism, such as a spring, to release the control cable and, hence, allow the derailleur to move the drive chain into central alignment with the desired sprocket. Details of a highly effective overshift return system are disclosed in Co-pending application Ser. No. 08/384,013 (Attorney Docket No. SRAM-01-002), filed Feb. 6, 1995.
During upshifting and overshift return, the control cable also slides axially with respect to the sheath. The cable tension moving the cable in the upshifting direction is limited by the force of the derailleur biasing spring. Thus, contamination, such as dirt or moisture, between the cable and the sheath can, and in many instances will, produce undesirable frictional forces. The frictional forces increase the force and time necessary to move the control cable. Accordingly, to optimize performance of the gear shift system, it is desirable to minimize such contamination and the resultant frictional forces.
Further, in general, the shift actuator produces "overtravel" of the control cable past the detented position corresponding to the target sprocket center. Hysteresis, a common phenomena in shifting systems, is exhibited in the cable linkage due to such factors as cable wire stretch, outer sheath compression, wear and/or excessive tolerances between the end cap and braze-on, end cap cable adjuster nut, and excessive wear and/or tolerances in the derailleur pivots. Since an increase in friction in the derailleur pivot and cable inner wire/sheath contact points increases the tension in the cable, the sheath compression and inner wire stretch will increase proportionally and, hence, increase the hysteresis.
A considerable portion of the cable overtravel produced by the shift actuator will be absorbed by the hysteresis. Overtravel above that which is absorbed by hysteresis will result in overshift at the derailleur. Overtravel at the shift actuator is typically 0.040 on a "high-end" product and up to 0.060 to 0.080 for a lower-end (large chain gap compatible) product. On a new, cleah high-end bike, the hysteresis absorbs about 0.020 of overtravel. This produces an observable overshift at the derailleur of 0.02 times the actuation ratio. Thus, if the actuation ratio is approximately 2:1, the derailleur moves about 0.04 inches past the new sprocket center during a downshift. As the bike accumulates dirt (and corrosion), the hysteresis increases to a point where there is no observable overshift. Indeed, observable "undershift" is exhibited if the hysteresis exceeds the overtravel (Overtravel at the shifter, overshift at the derailleur).
On a low-end bike, the initial overtravel at the shifter is approximately 0.06 to 0.08 inches. On a new, low-end bike, the hysteresis is about the same as on a high-end bike, approximately 0.02 to 0.03. Thus, if the overtravel is approximately 0.06 and the hysteresis is 0.02, then the derailleur overshifts 0.04 times the actuation ratio or approximately 0.08.
In operation, it is actually desirable for the derailleur to hesitate in the overshifted position to assure that the shift occurs. Obviously, this is only important during downshifting. Hesitation, or duration is desirable as long as the derailleur returns to sprocket center reliably after the shift is completed. As the low-end bike accumulates dirt and becomes contaminated, the hysteresis increases and absorbs more of the overtravel. It can therefore be seen that there is a need for a device that prevents the hysteresis from increasing due to contamination and assists the derailleur biasing spring in pulling the control cable toward the derailleur for quick upshifting.
Control cable contamination can also corrode the control cable, further increasing friction between the control cable and the sheath. In addition, such friction will increase control cable wear and can cause premature failure of components connected with the control cable.
To reduce control cable contamination, several manufacturers have attempted to seal the entrance of the control cable sheath with a dynamic seal. A typical dynamic seal includes a sheath having end cap and an o-ring. In operation, the o-ring attaches internal to the end cap and circumscribes the control cable; the end cap attaches over the sheath entrance.
Dynamic seals have several significant drawbacks. For example, friction is generated between the seal and the internal control cable. Further, movement of the control cable into the sheath can, and in most instances will, carry moisture and dirt into the sheath. A need therefore exists for a means of inhibiting contamination of the control cable while minimizing frictional forces associated with the movement of the control cable.
It is therefore an object of the present invention to provide a control cable pre-load and seal system for use with a bicycle derailleur gear shifting system%
It is another object of the present invention to provide a means for inhibiting contamination of a control cable while minimizing frictional forces associated with the movement of the control cable.
It is yet another object of the present invention to provide a control cable seal and pre-load system having a minimum number of components and readily adaptable on conventional derailleur-actuated multiple-gear bicycles.